here to access the pre-lab questions for this lab.
IntroductionOver hundreds or even thousands of years, humans have altered various species of plants and animals for our own use by selecting individuals for breeding that possessed certain desirable characteristics, and continuing this selective breeding process generation after generation. Thus, all domestic dog varieties, from chihuahuas to great danes, trace their separate lineages to a common wild ancestor, the wolf (Canis lupus). Similarly, a wide variety of familiar and highly nutritious vegetables originate from just a few species of wild mustards, in particular Brassica rapa, B. oleracea, and B. juncea.
That plant and animal breeders have been able to dramatically change the appearance of various lineages of organisms in a relatively short period of time is an obvious yet profound fact. Charles Darwin used many examples of selection by humans to help support the case for his proposed mechanism resulting in evolution of natural populations; i.e., natural selection.
Many studies on many kinds of organisms have shown that most variable traits respond to artificial selection (i.e., it is usually possible, even easy, to increase or decrease the frequency or average value of a trait in a lineage through careful selective breeding). Starting today in a lab exercise, you will attempt to accomplish the same thing.
Artificial selection vs. natural selectionNatural selection is a deceptively simple concept, relatively easy to understand at a basic level, but with profound implications that are intellectually challenging. The following exercise in artificial selection will serve as an introduction to natural selection, and we hope will help you to better understand this important concept. Fundamentally, artificial selection and natural selection are quite similar, but there are a few important differences.
The definition of natural selection is simply differential survival and reproductive success. In other words, not all individuals in a population leave the same number of offspring. This arises because individuals vary in many traits that affect their ability to survive and reproduce in a given environment. When this variability has a genetic basis (i.e., is inheritable-passed on from parents to offspring), natural selection can lead to evolutionary change in the expression of the trait in the population.
Artificial selection is essentially this same process, except that favored traits are those that for one reason or another are preferred by humans, rather than those that enhance the organism's fit to its environment. Artificial selection generally is much faster than natural selection, because the next generation can be absolutely restricted to offspring of parents that meet the desired criteria (rarely is natural selection such an all-or-none phenomenon).
An Artificial Selection Experiment Using Wisconsin Fast PlantsNormally, natural selection, and even artificial selection, occurs much too slowly to directly observe in a classroom setting. However, Wisconsin Fast Plants, a variety of Brassica rapa (the same species as turnip, bok choi and Chinese cabbage) has a remarkably short life cycle (6-7 weeks from seed to seed under lab conditions), which should let us study one complete generation this semester. Fast Plants are a product of 20 years of intense artificial selection by researchers at the University of Wisconsin for the following traits: rapid flowering and maturation; high seed production; short stature; and the ability to thrive under artificial light. The result of these efforts has been a valuable research and educational tool.
The lab you set up today will run much of the length of the semester. The basic goal of this lab is to see whether intense artificial selection on a single trait in a parental generation will cause that trait to evolve in the offspring generation. Today you will plant the seeds of the parental generation of Fast Plants, all from the same basic stock of "wild type" Fast Plants obtained from a commercial source.
Acknowledgements: This lab was derived from one developed by Dr. Bruce A. Fall of the University of Minnesota.